When a multi-cylinder engine is used, an intake manifold with the same number of intake passages as the cylinders is disposed between the engine and the throttle body. As disclosed in Japanese Patent Laid-open 2001-342917 (page 3 and FIG. 4 through 5), intake manifolds made of synthetic resin are utilized from the points of easiness in forming shapes, lightening, and cost reduction etc., since various shapes of intake passages are formed in an intake manifold.
An intake manifold made of synthetic resin is explained in the following. As shown in FIG. 16, an intake manifold 10 comprises three members; a lower member 14 connecting to a throttle body 12, a middle member 18 (see FIG. 17) as a first member connecting one end to the lower member 14 and the other end to an engine 16, and an upper member 20 (see FIG. 18) as a second member connecting to the upper side of the middle member 18. As shown in FIG. 19, four intake passages 22a, 22b, 22c, 22d, for example, are formed in the intake manifold 10.
As shown in FIG. 17 and FIG. 20, the middle member 18 has four branched lower arms 24a, 24b,24c, 24d which are shaped like, for example, a pipe cut in half in the axis direction and bended in a desired form. As shown in FIG. 20, a pair of mount connecting faces 26a-1, 26a-2 is formed at both sides of the branched lower arm 24a in the vicinity of the engine 16. Then, a lower passage space 30a is formed as a hollow between the pair of the mount connecting faces 26a-1, 26a-2. This lower passage space 30a forms approximately the lower half of the space of the intake passage 22a. In the same manner, a pair of mount connecting faces 26b-1, 26b-2 is formed at both sides of the branched lower arm 24b, and a lower passage space 30b is formed as a hollow between the pair of the mount connecting faces 26b-1, 26b-2. In the same manner, a pair of mount connecting faces 26c-1, 26c-2 is formed at both sides of the branched lower arm 24c, and a lower passage space 30c is formed as a hollow between the pair of the mount connecting faces 26c-1, 26c-2. In the same manner, a pair of mount connecting faces 26d-1, 26d-2 are formed at both sides of the branched lower arm 24d, and a lower passage space 30d is formed as a hollow between the pair of the mount connecting faces 26d-1, 26d-2. The lower intake passage spaces 30a, 30b, 30c,30d respectively form a part (which is the first intake passage) of the intake passages 22a, 22b, 22c, 22d. Further, as shown in FIG. 20, the mount connecting face 26a-2 and the mount connecting face 26b-1 become a mount connecting face 28ab at some midpoint. The mount connecting face 26b-2 and the mount connecting face 26c-1 become a mount connecting face 28bc at some midpoint. The mount connecting face 26c-2 and the mount connecting face 26d-1 become a mount connecting face 28cd at some midpoint.
As shown in FIG. 18, the upper member 20 has four branched upper arms 32a, 32b, 32c, 32d which are shaped like, for example, a pipe cut in half in the axis direction and bended in a desired form. The upper passage spaces 34a, 34b, 34c, 34d are respectively formed at the branched upper arms 32a, 32b, 32c, 32d as hollows. The upper passage spaces 34a, 34b, 34c, 34d respectively form the second intake passages which is approximately the upper half of the intake passages 22a, 22b, 22c, 22d. 
After respectively connecting the branched lower arms 24a, 24b, 24c, 24d of the middle member 18 shown in FIG. 17 and the branched upper arms 32a, 32b, 32c, 32d of the upper member 20 shown in FIG. 18, vibration welding is performed at the connecting positions. Then, each lower passage space 30a, 30b, 30c, 30d and each upper passage space 34a, 34b, 34c, 34d are connected, and four intake passages 22a, 22b, 22c, 22d shown in FIG. 19 are formed.
The connecting state of the branched lower arm 24 and the branched upper arm 32 is shown in FIG. 21. The connecting face 26 (28) of the branched lower arm 24 with the branched upper arm 32 has a center connecting face 38 which is a recess connecting face, and a side connecting face 40 which is a convex connecting face located at both sides of the center connecting face 38. The connecting face 26 (28) corresponds to the mount connecting faces 26a-1, 26a-2, 26b-1, 26b-2, 26c-1, 26c-2, 26d-1, 26d-2, 28ab, 28bc, 28cd shown in FIG. 20. On the other hand, the connecting face 42 of the branched upper arm 32 to the branched lower arm 24 has a center connecting face 44 which is a convex connecting face, and a side connecting face 46 which is a recess connecting face located at both sides of the center connecting face 44. The center connecting face 38 of the branched lower arm 24 and the center connecting face 44 of the branched upper arm 32 are arranged to be in the same matching plane when connected. Further, the side connecting face 40 of the branched lower arm 24 and the side connecting face 46 of the branched upper arm 32 are arranged to be in the same matching plane when connected.
The branched lower arm 24 and the branched upper arm 32 are fixed by vibration welding by contacting the center connecting face 38 of the branched lower arm 24 with the center connecting face 44 of the branched upper arm 32, and contacting the side connecting face 40 of the branched lower arm 24 with the side connecting face 46 of the branched upper arm 32. In this manner, the connecting face 26 (28) of the branched lower arm 24 and the connecting face 42 of the branched upper arm 32 are welded and fixed. Then, the intake passages 22a, 22b, 22c, 22d are formed. Here, the recess center connecting face 38 and the convex side connecting face 40 can be formed not at the branched lower arm 24 but at the branched upper arm 32, and the convex center connecting face 44 and the recess side connecting face 46 can also be formed not at the branched upper arm 32 but at the branched lower arm 24. In the following explanation, the connecting position of the branched lower arm 24 and the branched upper arm 32 is indicated by one plane for convenience.
As shown in FIG. 20, an attachment base 48 is integrally formed at the engine 16 side of the middle member 18. Four cylinder-shaped bores 50a, 50b, 50c, 50d are formed at the attachment base 48. The four bores 50a, 50b, 50c, 50d connect one side to the engine 16 and the other side to the lower passage space 30a, 30b, 30c, 30d. Namely, the bores 50a, 50b, 50c, 50d respectively form a part (which is the first intake passage) of the intake passages 22a, 22b, 22c, 22d. Here, the axis centers of the cylindrical bores 50a, 50b, 50c, 50d are arranged to be in parallel and numbered as 52a, 52b, 52c, 52d. Further, all the axis centers 52a, 52b, 52c, 52d are arranged to intersect with a line in the standard direction for vibration (line A-A in FIG. 20). The standard direction for vibration means the direction in which welding vibration is applied. For example in FIG. 16, it is the direction perpendicular from the front side of the paper towards the back side and vice versa. Line A-A shows one line in the standard direction for vibration.
Respective traveling direction lines of the lower passage spaces 30a, 30b, 30c,30d towards the bores 50a, 50b, 50c, 50d are shown by Ra, Rb, Rc, Rd in FIG. 20. In the intake manifold 10, the directions Ra, Rb, Rc, Rd of the intake passages 22a, 22b, 22c, 22d (the lower passage space 30a, 30b, 30c, 30d) towards the respective bore 50a, 50b, 50c, 50d are not aligned because of layout restrictions. Therefore, only one of the four intake passages 22a, 22b, 22c, 22d, namely, the intake passage 22a, can be arranged in the ideal direction. Then, the traveling direction line Ra of one lower passage space 30a out of the lower passage spaces 30a, 30b, 30c, 30d which are formed in the middle member 18 is arranged to be perpendicular to line A-A in FIG. 20. That is, an ideal arrangement. The rest of the traveling direction lines Rb, Rc, Rd of the lower passage spaces 30b, 30c, 30d are arranged so that as the distance from the traveling direction line Ra increases, the respective intersecting angle with line A-A gradually becomes smaller than 90 degrees.
As shown in FIG. 20, the approximately half-ring-shaped top end connecting faces 54a, 54b, 54c, 54d which position on the same plane are formed around each bore 50a, 50b, 50c, 50d. The top end connecting face 54a is connected at both ends to the mount connecting faces 26a-1, 26a-2. In the same manner, the top end connecting face 54b is connected to the mount connecting faces 26b-1, 26b-2. The top end connecting face 54c is connected to the mount connecting faces 26c-1, 26c-2. The top end connecting face 54d is connected to the mount connecting faces 26d-1, 26d-2.
Rising boundary lines from the top end connecting face 54a to the mount connecting faces 26a-1, 26a-2 are indicated as 56a-1, 56a-2. In the same manner, rising boundary lines from the top end connecting face 54b to the mount connecting faces 26b-1, 26b-2 are indicated as 56b-1, 56b-2. Rising boundary lines from the top end connecting face 54c to the mount connecting faces 26c-1, 26c-2 are indicated by 56c-1, 56c-2. Rising boundary lines from the top end connecting face 54d to the mount connecting faces 26d-1, 26d-2 are indicated as 56d-1, 56d-2.
As mentioned above, all the connecting portions of each branched lower arm 24a, 24b,24c, 24d and each branched upper arm 32a, 32b, 32c, 32d are arranged to be parallel to line A-A in FIG. 20, namely to the standard direction for vibration. Therefore, conventionally, all the rising boundary lines 56a-1, 56a-2, 56b-1, 56b-2, 56c-1, 56c-2, 56d-1, 56d-2 are arranged to be on the same line as line A-A in FIG. 20.
Since the mount connecting face 26a-1, 26a-2 which is connected to the top end connecting face 54a is disposed to be perpendicular to line A-A in FIG. 20, the ridge line 58a at the inside of the mount connecting face 26a-1, 26a-2 towards the lower passage space 30a does not project to the lower passage space 30a side. However, the ridge line 58b at the inside of the mount connecting face 26b-2 projects to the lower passage space 30b side. In the same manner, the ridge line 58c at the inside of the mount connecting face 26c-2 projects to the lower passage space 30c side. The ridge line 58d at the inside of the mount connecting face 26d-2 projects to the lower passage space 30d side.
The connecting face 26 of the branched lower arm 24d of the middle member 18 and the connecting face 42 of the branched upper arm 32d of the upper member 20 are connected as shown in FIG. 21. FIG. 22 is a sectional view at line X-X in FIG. 20 in the state that the connecting faces 24, 42 are vibration-welded. Line X-X is arranged to be inclined to line A-A. In FIG. 23, the branched lower arm 24d of the middle member 18 and the branched upper arm 32d of the upper member 20 in FIG. 22 are shown in a separated manner. In FIG. 23, in the case that the branched lower arm 24d of the middle member 18 is formed by a die, the die is pulled out in the direction of arrow Z1. The ridge line 58d at the inside of the upper face of the mount connecting face 26d-2 projects to the lower passage space 30d side and beyond a dent position 59 which is the most dented position of the inner wall of the lower passage space 30d. Therefore, the lower region of the ridge line 58d cannot be scooped out to the dent position 59. Thus, a lower thick portion 60d is formed below the ridge line 58d. The lower thick portion 60d, which is a sectional portion slashed by dotted lines in the figure, is the area surrounded by a vertical line 62 drawn from the ridge line 58d and an arc 64 shown by a dotted line. It is ideal that the arc 64 becomes the wall face of the lower passage space 30d. In the same manner, in the case that the branched upper arm 32d of the upper member 20 is formed by a die, the die is pulled out in the direction of arrow Z2. In this case also, because of the same reason as the branched lower arm 24d, an upper thick portion 66d, which is the sectional portion slashed by dotted lines in the figure, is formed at the branched upper arm 32d. 
In the state shown in FIG. 22, the lower thick portion 60d formed at the branched lower arm 24d and the upper thick portion 66d formed at the branched upper arm 32d project towards the inside of the intake passage 22d. As a result, because of the lower thick portion 60d and the upper thick portion 66d, the sectional shape of the intake passage 22d cannot become circular which is the ideal shape. Here, since respective angles of the traveling direction lines Ra, Rb, Rc, Rd of the lower passage spaces 30a, 30b, 30c, 30d of the branched lower arms 24a, 24b, 24c, 24d against line A-A each differ, the sectional shapes of the intake passages 22a, 22b, 22c, 22d each differ as well. For example, when the section of the intake passage 22a is formed as an ideal circular shape, the sectional shapes of the intake passage 22b, the intake passage 22c, and the intake passage 22d gradually become distorted.
In FIG. 23, the connecting face 26d-1, 26d-2 of the branched lower arm 24d appears as an inclined state. The section at line X-X in FIG. 20 is inclined against line A-A. Therefore, according to the inclined angle, the connecting face 26d-1, 26d-2 appears as an inclined state against the horizontal line.
The connecting face 26d-1, 26d-2 of the branched lower arm 24d and the connecting face 42 of the branched upper arm 32d are connected as shown in FIG. 21. FIG. 24 is a sectional view at line Y-Y in FIG. 20 in the state that the connecting faces are vibration-welded. Line Y-Y is parallel to line A-A. As can be seen in FIG. 24, the lower thick portion 60d of the branched lower arm 24d and the upper thick portion 66d of the branched upper arm 32d project to the inside of the intake passage 22d, and the section of the intake passage 22d is narrowed. Further, in the branched lower arm 24d, the connecting portion for connecting to the upper thick portion 66d of the branched upper arm 32d is thickened. In the branched upper arm 32d, the connecting portion for connecting to the lower thick portion 60d of the branched lower arm 24d is thickened. Since line Y-Y is parallel to line A-A, both of the two connecting faces 26d-1, 26d-2 of the branched lower arm 24d position on the horizontal line H-H.
As can be seen in FIG. 22 and FIG. 23, the intake passages 22b, 22c, 22d cannot be formed as an ideal circular sectional shape like the intake passage 22a. The reason is as follows. The lower thick portion 60 is formed at the branched lower arm 24b, 24c, 24d by die molding of the middle member 18. The upper thick portion 66 is formed at the branched upper arm 32b, 32c, 32d by die molding of the upper member 20. The lower thick portion 60 and the upper thick portion 66 project to the inside of the intake passage 22b, 22c, 22d. Consequently, the sectional shapes of the intake passages 22b, 22c, 22d each differ and cannot be circular. Thus, the intake air amount of each intake passage 22b, 22c, 22d cannot be evenly obtained. Therefore, there is a problem that desired engine performance cannot be obtained.
In FIG. 20 and FIG. 23, a plurality of intake passages 22a, 22b, 22c, 22d is inclined respectively in a different angle. When one intake passage 22a is formed to have an ideal circular sectional shape against line A-A which is in the standard direction for vibration, the sections of the rest of the three intake passages 22b, 22c, 22d cannot be the ideal shape. In the case that only one intake passage is disposed, when it is not arranged in a specific angle against line A-A which is in the standard direction for vibration, the ideal sectional shape cannot be obtained.